Dragons make everything Better:
i used emerald proto whelp 2-2-2 stormborne whelping 1 - 2-2 and emerald whelping 2-2-2 which i didn't need with the first pet just make sure you keep the healing up and you should be fine..
I tried this strat like 6-7 times before I gave up. Hopling never gets that second backflip off, and Terky only gets the second enemy down to half health, so there's no Stampede buff on the third enemy and it's a fail every time.
I don't see Feral Vermin as a valid substitute here; he's too slow (289 vs 325 for hopling) so backflip accomplishes very little. A Flayer Youngling S/S with kick might work, although he doesn't have a strong critter attack. Mine are S/B and H/S so can't test it.
This worked first try after I had horrible luck with the one DragonAfterDark posted with the Father Winter's small helpers :) Made an account just to upvote this strategy - thanks for figuring out how to neutralize the undead rounds!